A radioimmunoassay of 3,5-dimethyl-3'-isopropyl-L-thyronine

A simple, sensitive, and specific RIA has been developed to measure 3,5-dimethyl-3'-isopropyl-L-thyronine (DIMIT) in unextracted serum. The RIA employs a DIMIT-binding antibody obtained from a rabbit immunized with DIMIT-human serum albumin conjugate and [125I]T3 as the radioligand. 8-Anilino-1-naphthalene sulfonic acid and barbital buffer (0.075 M; pH 8.6) were used to minimize serum binding of DIMIT and [125I]T3. The detection threshold of the RIA was 0.03 microgram/dl. Recovery of nonradioactive DIMIT added to serum averaged 104%. 3,5,3'-Triiodothyroacetic acid cross-reacted maximally (78%) in the RIA, followed by T3 (56%), 3,5-diethyl-3'-isopropyl thyronine (43%), 3,3'-diiodothyronine (T2; 12%), tetraiodothyroacetic acid (6.7%), 3'-monoiodothyronine (T1; 0.54%), 3.5-T2 (0.34%), 3-T1 (0.26%), and T4 (0.25%); rT3, 3',5'-T2, thyronine, iodotyrosines, and KI cross-reacted 0.1% or less. The coefficient of variation averaged 7.4% within an assay and 9.0% between assays. After ip administration of DIMIT to the rat, the mean serum DIMIT concentration rose in a dose-related fashion from less than 0.12 microgram/dl in the controls to 44 micrograms/dl in those receiving 200 micrograms/100 g BW daily for 3 days; serum T4 and TSH levels declined significantly in animals receiving 3.2 micrograms DIMIT/(100 g BW X day) or more. After a single ip injection of DIMIT to the rat, serum DIMIT levels increased from a control value of 0.04 to 9.8 micrograms/dl at 2 h and then decreased progressively, with a half-life approximating 7.2 h. Serum TSH was suppressed for up to 24 h, while serum T4 and T3 were suppressed for up to 48 h after the injection. Concentrations of DIMIT 15 h after a dose of 10 micrograms, ip, to approximately 150-g rats were (nanograms per g; mean +/- SE) 19 +/- 2.3 in serum, 30 +/- 3.5 in liver, 23 +/- 2.4 in kidney, 19 +/- 4.7 in heart, 16 +/- 3.4 in skeletal muscle, and 20 +/- 3.0 in brain. DIMIT administration to pregnant rats [3 micrograms/(100 g BW X day) for 3 days, im] resulted in mean +/- SE DIMIT concentrations (micrograms per dl) of 0.51 +/- 0.14 in maternal serum, 0.57 +/- 0.04 in fetal serum, and 0.46 +/- 0.07 in amniotic fluid. A DIMIT dose of 10 micrograms/(100 g BW X day) for 3 days in maternal rats resulted in mean DIMIT concentrations (micrograms per dl) of 1.8 +/- 0.06 in maternal serum, 1.4 +/- 0.10 in fetal serum, and 1.3 +/- 0.17 in amniotic fluid.(ABSTRACT TRUNCATED AT 400 WORDS)